fix: Fixed width for dataset panel

[kgabryje]

SUMMARY

This PR sets a fixed width for dataset panel on Explore view - 320px. The goal was to prevent resizing when datasource changes and it's name is much shorter or longer than the previous one.
Closes #12142

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Nagranie.z.ekranu.2020-12-29.o.10.30.48.mov

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue: [explore] data panel should have fixed width  #12142
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

CC: @junlincc @etr2460

[kgabryje]

@junlincc The issue doesn't specify the width, so I set it to 320px. I can change it of we want different value

[junlincc]

in this PR, both data and control panels are 320px, which look perfectly fine to me, while they are supposed to be 288px according to @mihir174 's the design proposal.
Because we are still at the intermediate stage where the control panel can not go down to 288px yet, it would look quite weird if two panels have different width. so let's keep both 320px for now.

manually tested, LGTM

Screen.Recording.2021-01-01.at.1.31.50.PM.mov

[ktmud]

LGTM

[codecov-io]

Codecov Report

Merging #12221 (f2174a7) into master (f3ab1f4) will increase coverage by 0.69%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #12221      +/-   ##
==========================================
+ Coverage   66.20%   66.89%   +0.69%     
==========================================
  Files         996     1002       +6     
  Lines       49174    49585     +411     
  Branches     4993     5090      +97     
==========================================
+ Hits        32554    33172     +618     
+ Misses      16476    16291     -185     
+ Partials      144      122      -22     

Flag	Coverage Δ
cypress	51.42% <100.00%> (+7.32%)	⬆️
javascript	61.22% <0.00%> (+0.10%)	⬆️
python	64.14% <ø> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...nd/src/explore/components/ExploreViewContainer.jsx	79.87% <100.00%> (+1.29%)	⬆️
...erset-frontend/src/SqlLab/components/ResultSet.tsx	44.54% <0.00%> (-22.28%)	⬇️
...ntend/src/explore/components/ExploreChartPanel.jsx	53.60% <0.00%> (-20.64%)	⬇️
superset-frontend/src/chart/Chart.jsx	53.08% <0.00%> (-20.13%)	⬇️
superset/commands/importers/v1/examples.py	34.11% <0.00%> (-11.34%)	⬇️
superset-frontend/src/components/Menu/Menu.tsx	83.17% <0.00%> (-10.80%)	⬇️
...explore/components/controls/AdhocFilterControl.jsx	55.44% <0.00%> (-10.21%)	⬇️
superset-frontend/src/components/Menu/NewMenu.tsx	80.00% <0.00%> (-4.62%)	⬇️
superset-frontend/src/components/Menu/SubMenu.tsx	97.61% <0.00%> (-2.39%)	⬇️
...perset-frontend/src/components/Menu/MenuObject.tsx	77.27% <0.00%> (-1.68%)	⬇️
... and 89 more

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